How many outriders would you need to make up an effective warband? Are they really worth having over regular riders ?

From the way I have seen them ( never used them though ) I would say not a full Warband. 3-6 Models is what I usually see and it seems like a good number. They are meant for harassing the enemy not full front combat! That's what your Riders are for. So to answer I would say a small portion but I'm no expert

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Kili - "Aren't you going to search me? I could have anything down my trousers"
Tauriel - "Or nothing"

One of the difficulties with outriders is that unlike regular riders they're not exempt from the bow limit.
I have run 3 at 500 pts in an army of 20 with Royal guards added to make the outriders legal.
My 600pt list also has three but 4 is really the maximum I'd go for at that kind of level.

While their shoot value is great you sacrfice shields and higher defence that the riders bring. Definately include a couple if you can but I wouldn't aim for large numbers of them.

The exception to this is if your going for infantry, the outrider is much better than a regular warrior with bow.